This invention pertains in general to surface wave devices. These are a class of well-known devices comprising a medium capable of propagating surface waves, an input or launching transducer for launching surface waves across a surface of the substrate, and an output or receiving transducer for receiving the propagated surface waves. Such devices have been developed to an advanced state and are now in commercial use.
This invention pertains particularly to novel surface wave devices having the property that the mechanical component of spurious waves reflected from a transducer boundary or internal irregularity are suppressed.
It has been known that an electrode array composed of a pair of interdigitated combs of conductive teeth at unlike potentials, if coupled to a piezoelectric medium, produces acoustic surface waves on the medium. In a simplified embodiment of a piezoelectric ceramic poled perpendicularly to the propagating surface, the waves travel at right angles to the teeth. The surface waves are converted back into electrical signals by a similar array of conductive teeth coupled to the piezoelectric medium and spaced from the input electrode array. In principle, the tooth pattern is analogous to an antenna array. Consequently, similar signal selectivity is possible, thereby eliminating the need for the critical or much larger and more cumbersome components normally associated with frequency-selective circuitry. Thus, such a device, with its small size, is particularly useful in conjunction with solid-state functional integrated circuitry where signal selectivity is desired.
The usual surface wave device has a finite distance between its input and output transducers. Hence, a finite time is required for an acoustic surface wave to travel along the path from the input transducer to the output transducer. At the output transducer, part of the acoustic-wave energy is converted to electrical energy and delivered to a load. Another part of the acoustic-wave energy is transmitted past the output transducer where it may be terminated or dissipated. A still further part of the arriving acoustic-wave energy is reflected back along the original path toward the input transducer. This reflected surface wave, which is smaller in magnitude than the original surface wave, intercepts the input transducer from which a portion of the wave again is similarly reflected back along the same path to the output transducer where it appears as a diminished replica of the original surface wave. Because of the additional distance of travel, the smaller version of the original surface wave arrives at the output transducer later than that original wave. The time delay is equal to twice the time required for a surface wave to transverse the path from the input transducer to the output transducer. When such a surface wave device is used, for example, as a signal-selective device in a television intermediate-frequency amplifier, the triple-transit reflected signal components appear as a ghost in the picture and makes it highly undesirable, if not completely unacceptable, for normal viewing.
The non-aimed end of a surface wave transducer is another source of spurious reflections. Intelligence-bearing surface waves launched by a transducer in the opposite direction from the receiving transducer reflect internally from the non-aimed end, propagate internally across the transducer, and emerge from the aimed end out of phase with the main waves. The internally reflected spurious boundary wave will appear as a ghost when received by the output transducer.
Still another source of spurious reflections are irregularities within the transducer tooth pattern in the form of radical changes in the width or spacing of the comb teeth.
Known methods for approaching the spurious wave problem have included optimizing the signal-transducing characteristics of one or both of the input and output transducers, depositing an attenuating material between the input and output transducers, and utilizing an additional transducer, spaced from the input and output transducers, responsive to a portion of the original surface wave for generating a still additional acoustic surface wave that at least partially counteracts the undesired acoustic wave orginally reflected back from the output transducer.
An improvement in the latter respect is disclosed and claimed in a patent to Adrian DeVries, U.S. Pat. No. 3,727,155, assigned to the same assignee as the present application. As there taught, reflection components, arising by reason of mechanical loading of the substrate by a transducer, and local electric field shorting caused by the transducer electrodes, are at least reduced by subdividing each "tooth" of the interdigitated combs into a pair of conductively connected ribbons spaced apart by one-fourth the acoustic wavelength. For best results, the DeVries transducer is connected to a source or load impedance significantly smaller than the impedance of the transducer itself. It is recognized in U.S. Pat. No. 3,723,419 by Robert Adler and assigned to the same assignee as the present invention, that at least under certain load conditions an additional contributor to the production of reflection components in a transducer of the DeVries type may be electrical loading of the substrate which occurs as a result of electrical shorts created by conductive bars that interconnect different ones of the ribbons. The approach described and claimed in the Adler application seeks to overcome such additional electrical loading by individually connecting the respective ribbons of each adjacent "tooth"-forming pair to correspondingly separate electrical loads. However, the necessity of associating the surface wave device with such plural loads and the attendant isolation schemes result in substantial complexity.
Yet another approach to eliminating triple-transit reflections is taught in U.S. Pat. No. 3,662,293-DeVries, assigned to the assignee of the present application. It is there taught that in order to inhibit the development of spurious reflections, a plurality of surface discontinuities such as grooves are formed in the wave propagating surface alongside the output transducer. These grooves reflect surface waves and are spaced from the input transducer by such a distance that surface waves reflected from the grooves reach the input transducer in a predetermined time and phase relationship with respect to acoustic surface waves reflected by the output transducer. At least partial cancellation of the spurious reflections results.
Still another approach is taught in U.S. Pat. No. 3,748,603 Wojcik, also assigned to the assignee of the present application. Wojcik discloses a surface wave device having input and output transducers disposed on a surface of a wave-propagative medium. One or both of the transducers includes a pair of interdigitated combs of conductive material disposed along the propagation path. Adjacent teeth of the combs are spaced apart by a center-to-center distance of one-half the acoustic center wavelength. Electrically isolated conductive ribbons are disposed individually between the teeth in respective different pairs of adjacent teeth. The center-to-center spacing between each of the ribbons and the ones of the teeth adjacent thereto is one-fourth the acoustic center wavelength. The Wojcik teaching results in at least partial cancellation of waves reflected internally from the regular pattern of teeth which constitutes the body of the transducer.
It is known that in a surface wave resonator, surface waves reflected from a periodic array of electrically isolated conductive teeth or ribbons will be 180.degree. out of phase with waves reflected from a periodic array of ribbons or teeth which are bussed together. See "Relations for Analysis and Design of Surface Wave Resonators" Matthaei et al, IEEE Transactions on Sonics, Vol. SU-L3, No. 2, March, 1976. See also "Reflective Arrays for SAW Resonators", P. S. Cross, 1975 Ultrasonics Symposium Proceedings, IEEE Cat. No. 75 CHO 994-4SU, "Properties of Reflective Arrays for Surface Acoustic Resonators", P. S. Cross, IEEE Transactions on Sonics, Vol. SU-23, No. 4, July, 1976, and "Reflections on Surface Waves from Periodic Discontinuities", C. Dunnrowicz et al, 1976 Ultrasonics Symposium Proceedings, IEEE Cat. No. 76 CHII20-5SU.
It is very significant that all of the approaches to reflected wave cancellation which rely on quarter wave spacing of reflecting boundaries, e.g., U.S. Pat. No. 3,662,293-DeVries or U.S. Pat. No. 3,748,603-Wojcik, effect theoretically perfect cancellation at the synchronous (center) frequency only.
Other U.S. patents which pertain to surface wave devices and in particular to cancellation of spurious reflections in surface wave devices are: U.S. Pat. No. 3,757,256-Whitehouse et al; U.S. Pat. No. 3,573,673-DeVries et al; U.S. Pat. No. 3,582,838-DeVries; U.S. Pat. No. 3,596,211-Dias et al; U.S. Pat. No. 3,559,115-DeVries; and U.S. Pat. No. 3,582,540-Adler et al.